Recently, as speeds of computer processors and networks are rapidly increasing, a lot of researches are actively progressing to develop efficient and lightweight parallel computing platforms using heterogeneous and networked computers. According to this technical trend, this paper designs and implements a message passing library called JMPI(Java Message Passing Interface) which complies with MPJ, the MPI standard specification for Java language. This library provides some graphic user interface tools to enable parallel computing environments to be configured very simply by their administrators and JMPI applications to be executed very conveniently. Especially, it is implemented as two versions based on two typical distributed system communication mechanisms, Socket and RMI. According to these communication mechanisms, the performance of each message passing system is evaluated by measuring its processing speed with respect to the increasing number of computers by executing three well-known applications. Experimental results show that the most efficient processing speedup can be obtained by increasing the number of the computers in consideration of network traffics generated by applications.